Absolutely, you can send in your PhD application before you conclude your masters degree. However, you need to be aware of the fact that you probably will not have all your scores available on your masters transcript and as a result of this, some schools may admit you with the condition that you attain a certain grade in your masters. If your masters is two years and you already have your grades for one year on your transcript, this will work to your advantage, as the admissions committee can also judge part of your academic performance during the masters. If however, you intend to abandon the masters program (some PhDs do not require masters degrees) we urge you to think carefully about this, as you might have to give compelling reasons for abandoning the masters degree. Overall, if you do not want a gap between your masters year and the start of the PhD, you can apply for a PhD before the masters degree is concluded. Your chances of being admitted are not necessarily affected if your application as a whole is compelling.